Venting Design for Pallet Molds: How ISM Prevents Gas Traps and Burn Marks

Venting Design for Pallet Molds: How ISM Prevents Gas Traps and Burn Marks

During injection molding, as molten plastic fills the cavity, air and gases must escape. If they cannot escape, they become trapped and compressed. Compressed gas heats up and can burn the plastic, creating black or brown burn marks on the pallet surface. Gas traps also cause short shots, weak weld lines, and surface defects.

For large pallet molds with complex geometry, venting is critical. At ISM, we design pallet mold venting systems that eliminate gas traps and burn marks. Here is how we do it.

1. Understanding the Problem: Gas Traps and Burn Marks

Where gas traps occur: Deep ribs, leg corners, fork entry tunnels, areas opposite gates, last points of cavity fill.

2. ISM's Venting Design Principles

A. Vent Location Strategy

ISM places vents at the last points of cavity fill as predicted by Moldflow simulation, at the ends of long flow paths such as pallet corners, at deep rib bottoms where air is pushed ahead of the melt front, around core pins and leg cores, and at weld line convergence points.

B. Vent Depth Guidelines

Vent depth is critical. Too shallow and vent does not work. Too deep and plastic flashes into the vent.

C. Vent Land Length

The vent land is the length of the vent channel at the specified depth. Land length should be 1.0 to 2.0 millimeters. Beyond the land, the vent can open to a deeper relief channel of 0.5 to 1.0 millimeters depth. This deeper section connects to atmosphere.

3. Types of Vents in Pallet Molds

A. Parting Line Vents

The most common vent type. Shallow grooves machined into the parting line surface. Best for flat areas and perimeter edges. ISM uses CNC ground vents for precision depth control.

B. Ejector Pin Vents

Ejector pins naturally have small clearances. ISM uses ejector pins as vents by placing them at gas trap locations. The pin clearance of 0.01 to 0.02 millimeters acts as a vent. No additional machining is required.

C. Insert Vents

For areas where parting line vents are not possible, such as deep ribs or leg cores. ISM uses vented inserts with micro-grooves cut into the insert surface. The insert is made of porous steel or has precision ground channels.

D. Vacuum-Assisted Venting (Advanced)

For extremely difficult pallet geometries, ISM offers vacuum venting. A vacuum pump pulls air from the cavity before injection. This eliminates all gas traps. It adds equipment cost but is effective for thin wall pallets with long flow paths.

4. Venting for Specific Pallet Features

A. Deep Ribs

Challenge: Air is trapped at the bottom of ribs. Solution: Place vent pins at the bottom of rib cores. Use parting line vents at rib ends. Increase rib draft to allow air to escape upward.

B. Leg Cores

Challenge: Air trapped inside leg cavities. Solution: Vented ejector pins through leg centers. Parting line vents around leg perimeters. Small vent holes in leg core sides if acceptable.

C. Fork Entry Tunnels

Challenge: Long narrow tunnels trap air. Solution: Multiple vent grooves along the tunnel length. Vent inserts at tunnel ends. Higher injection speed to push air ahead of melt.

D. Corners

Challenge: Last point of fill for many pallets. Solution: Large parting line vents at all four corners. Relief channels to atmosphere. Vacuum assist for thin wall designs.

5. Simulation for Venting Design

Before building the mold, ISM uses Moldflow simulation to predict gas trap locations.

Simulation outputs: Air trap locations shown as colored regions. Weld line positions for vent placement verification. Fill pattern to identify last fill points. Pressure distribution to check back pressure from trapped air.

ISM standard: Any gas trap predicted by simulation must have a vent within 10 millimeters. No unvented gas traps are permitted.

6. Case Study: Large 1200x1000mm Pallet with Burn Mark Issues

Customer problem: Existing pallet mold produced burn marks at the four corners and at the center of the top deck. Scrap rate was 8 percent due to burn marks. Material was HDPE with 20 percent talc. The mold had minimal venting with only four shallow parting line vents.

ISM venting redesign

First, Moldflow simulation identified gas traps at all four corners and along a weld line across the top deck. The ISM solution added 2.0 millimeter wide vents with 0.035 millimeter depth and 2.0 millimeter land length at all four corners. Additional vents were placed along the predicted weld line. Four ejector pins were positioned as vent pins at high trap locations. Relief channels of 0.8 millimeter depth were machined from vents to the mold edge.

Results

Burn marks were completely eliminated. Scrap rate dropped from 8 percent to 1.2 percent. Cycle time was unchanged because venting does not affect cooling. The customer reported annual savings of 40,000 USD from reduced scrap.

7. Case Study: Deep Rib Pallet for Heavy Duty Use

Customer problem: A pallet with 40 millimeter deep ribs showed burn marks at the bottom of every rib. Scrap rate was 12 percent. The previous mold had no vents at rib bottoms. Material was glass filled polypropylene, which is more abrasive and requires deeper vents.

ISM venting solution

Vented ejector pins were installed at the bottom of each rib. Each pin had 0.02 millimeter clearance for venting. Parting line vents were added at the ends of all ribs. Vent depth was set to 0.045 millimeters for glass filled material.

Results

Burn marks at rib bottoms were eliminated. Scrap rate dropped to 1.5 percent. No flash occurred because vent depth was correctly set for glass filled material. The mold required no other modifications.

8. Vent Maintenance

Vents clog over time with plastic residue and dust. ISM recommends regular vent cleaning.

Warning: Never use steel tools to clean vents. Steel scratches the vent land, increasing vent depth and causing flash.

9. Common Venting Mistakes

10. Signs Your Pallet Mold Has Insufficient Venting

Visible burn marks are black or brown streaks on the pallet surface, especially at corners or rib bottoms. Short shots occur where melt stops before filling the cavity. Glossy spots or dull areas may indicate gas trapped against the cavity surface. High injection pressure is needed to overcome back pressure from trapped gas. Sizzling or popping sounds during injection indicate gas escaping explosively.

If you see any of these signs, contact ISM for a venting audit.

11. Venting Checklist for New Pallet Molds

When ordering a new pallet mold from ISM, we verify that Moldflow simulation has identified all gas trap locations. Vents are placed within 10 millimeters of every predicted gas trap. Vent depth is specified based on the exact material you will use. Relief channels are machined from every vent to the mold edge. Ejector pins are positioned as vent pins where needed. All vents are documented in a vent map provided with the mold.

12. Retrofit: Adding Vents to Existing Molds

If you have an existing pallet mold with burn mark problems, ISM can add vents.

ISM recommendation: Start with parting line vents and vented ejector pins. If burn marks persist, consider vented inserts or vacuum assist.

Conclusion

Gas traps and burn marks are preventable. The solution is proper venting design based on simulation, material-specific vent depths, strategic placement at last fill points, and regular maintenance.

At ISM, we design pallet mold venting systems that eliminate burn marks, reduce scrap rates, and improve part quality. Every mold we ship includes a vent map and cleaning instructions.

Contact ISM today to discuss venting for your pallet mold project. We will provide a gas trap simulation and venting plan before we build your mold.